When an oil well is first drilled and completed, the fluids (such as crude oil) may be under natural pressure that is sufficient to produce on its own. In other words, the oil rises to the surface without any assistance.
In many oil wells and in particular those in fields that are established and aging, natural pressure has declined to the point where the oil must be artificially lifted to the surface. A subsurface pump is located down in the well below the level of the oil. A string of sucker rods extends from the pump up to the surface to a pump jack device, or beam pump unit. A prime mover, such as a gasoline or diesel engine, or an electric motor, or a gas engine, on the surface causes the pump jack to rock back and forth, thereby moving the string of sucker rods up and down inside of the well tubing.
The string of sucker rods operates the subsurface pump. A typical pump has a plunger that is reciprocated inside of a barrel by the sucker rods. The barrel has a standing one-way valve, while the plunger has a traveling one-way valve, or in some pumps the plunger has a standing one-way valve, while the barrel has a traveling one-way valve. Reciprocation charges a compression chamber between the valves with fluid and then lifts the fluid up the tubing toward the surface.
In the operation of reciprocating the pump, the sudden reversal of movement at the end of the upstroke and the downstroke of the sucker rods imposes reversal strains on the sucker rods as the stretch of the sucker rods is either suddenly released or suddenly imposed upon the rods.
In addition, the subsurface pump used in connection with the sucker rods can undergo what is known as gas lock. This is a condition which occurs when gas enters the compression chamber. The plunger cannot compress the gas to a pressure sufficient to force the traveling valve open. As the plunger is reciprocated, the gas inside the compression chamber is compressed and expanded.
In the prior art, one way to compensate for gas lock is to space the plunger so that it bumps the bottom on every downstroke in order to eliminate gas lock. This action of bumping the bottom causes destructive effects. It increases the stress range on the sucker rods. It causes the rods to go into the compression state each and every time the pump bumps the bottom. It also causes the rods to buckle and slap inside of the tubing, which causes increased wear to the rods, rod couplings and tubing. When the pump bumps bottom it causes the entire weight of the rods to be transferred to the tubing string in a shock load, which can cause premature failure of tubing couplings and threads. Such shock loads are also transferred to the pumping unit when the pump bumps bottom resulting in premature failure of the structural bearings and torque reversals in the gears in the gear box causing excessive wear on the gear teeth and gear box bearing.
Another condition is known as “fluid pound” and occurs in situations where the compression chamber is partially filled with liquid and gas. As the plunger moves on the downstroke through the gas it encounters the interface with the fluid and severely jars the sucker rods in the pump. A similar condition is called a “gas pound” and occurs when the plunger on the downstroke compresses gas to a pressure greater than the rod weight but not sufficient pressure to open the traveling valve. Unintentional gas or fluid pounding in operation of conventional pumps is a common problem in low fluid level wells and marginal producing wells.
U.S. Pat. No. 4,963,078 introduced a stress and torque reducing tool. The stress and torque reducing tool is located between the downhole pump and the sucker rod string and employs a plunger to effectively isolate the sucker rod string from the shock forces of the downhole pump. The plunger is housed in a body, which body has ports located above and below the plunger. The ports allow the entry and exit of well fluid so as to dampen the reciprocal movement of the plunger inside of the body.
While the stress and torque reducing tool of the '078 patent is effective in reducing stress and torque, it suffers from being difficult to retrieve. Due to the severe stresses and torque suffered by the tool, it occasionally fails by breaking. When the tool breaks, it is difficult to retrieve, or “fish” the tool from downhole. This is because there is not enough space to accommodate a fishing tool. The outside diameter of the stress and torque reducing tool is too large relative to the inside diameter of the tubing to allow passage of the fishing tool.